1. Field of Technology
The present invention generally relates to a subminiature coil assembly and, more particularly, to a multi-layer chip coil comprised of a stack of magnetizeable laminas having respective patterned conductors connected electrically in series with each other in a substantially zigzag fashion.
2. Description of the Prior Art
The multi-layer chip coil, precisely speaking, a laminated chip inductor, of a type generally comprised of a stack of magnetizeable laminas having respective patterned conductors formed thereon and connected electrically in series with each other in substantially zigzag fashion is disclosed in, for example, Japanese Laid-open Utility Model Publication No. 57-100209. This prior art multi-layer chip inductor is substantially reproduced in FIGS. 1 and 2 of the accompanying drawings for the purpose of detailed discussion of the prior art considered pertinent to the present invention, reference to which will now be made.
Referring to FIG. 1 showing the multi-layer chip inductor in exploded view, the prior art multi-layer chip inductor comprises a pair of generally rectangular outermost laminas 1 and 1' of magnetizeable material, each having first and second surfaces opposite to each other, and a plurality of intermediate laminas 2 of magnetizeable material stacked one above the other and firmly sandwiched between the outermost laminas 1 and 1', each of said intermediate laminas 2 having first and second surface opposite to each other. Only the first surfaces of the intermediate laminas 2 are respectively formed with generally U-shaped electric conductors 3, each opening towards one of the opposite ends of the associated intermediate lamina 2 and having its opposite ends 3a and 3b both terminating at a portion of the associated intermediate lamina 2 generally intermediate of the length thereof.
Each of the intermediate laminas 2 has defined therein a through-hole 4 extending completely through the thickness of the respective intermediate lamina 2 while opening at one end through one end 3a of the associated U-shaped conductor 3, which through-hole 4 has been formed by perforating the respective intermediate lamina 2 across the thickness thereof so as to leave a cylindrical wall coaxial with such through-hole 4. As best shown in FIG. 2 which illustrate the neighboring intermediate laminas in side sectional view, each cylindrical wall defining the respective through-hole 4 is plated, or otherwise lined, with the same electroconductive material as that for any one of the U-shaped conductors 3 so as to provide a conductive bushing 5 having one end continued to the end 3a of the associated U-shaped conductor 3 and the other end terminating flush the second surface of the respective intermediate lamina 2.
The outermost lamina 1 has its first surface formed with a generally L-shaped electric conductor 6 including a lengthwise strip 6a extending along and adjacent to one side of the lamina 1 and terminating at a portion thereof generally intermediate of the length of the lamina 1, and a lateral strip 6b continued to the lengthwise portion 6a and extending along one end thereof for electric connection with an end cap or like terminal member (not shown). Similarly, the outermost lamina 1' has its first surface formed with a generally L-shaped electric conductor 6' including a lengthwise strip 6'a extending along and adjacent to one side of the lamina 1' and terminating at a portion thereof generally intermediate of the length of the lamina 1', and a lateral strip 6'b continued to the lengthwise strip 6'a and extending along one end thereof for electric connection with another end cap or like terminal member (not shown). The outermost laminas 1 and 1' are similar in structure to each other except that only the outermost lamina 1' is formed with a through-hole 4' so as to extend completely through the thickness thereof while opening at one end through a free end portion of the lengthwise strip 6'a in a manner similar to the through-hole 4 in any one of the intermediate laminas 2, the cylindrical wall defining the through hole 4' being similarly plated or lined with the same electroconductive material to provide a conductive bushing (not visible) which has one end continued to the lengthwise strip 6'a and the other end terminating flush with the second surface of the outermost lamina 1' in a manner similar to the conductive bushing 5 as shown in FIG. 2.
When assembling the laminas 1, 2 and 1' together to provide a substantially complete multi-layer chip inductor, the intermediate laminas 2 are stacked one above the other with all of the first surfaces thereof oriented in one and the same direction while one intermediate lamina 2 is turned 180.degree. about the imaginary common axis, extending through all of the intermediate laminas 2 at right angles thereto, relative to the next adjacent intermediate lamina 2 positioned immediately thereabove or therebelow whereby the perforated end 3a of the conductor 3 in such one intermediate lamina 2 is aligned, and connected electrically through the associated conductive bushing 5, with the non-perforated end 3b of the conductor 3 in such next adjacent intermediate lamina 2. With respect to the outermost laminas 1 and 1', the laminas 1 and 1' are turned 180.degree. about the imaginary common axis referred to above, relative to each other, where the number of the intermediate laminas 2 is of an even-numbered value such as shown, so that the conductor 6 on the lamina 1 held in contact with the neighboring intermediate lamina 2 can be electrically connected with the conductor 3 on such neighboring lamina 2 through the conductive bushing 5 and, similarly, the conductor 6' on the lamina 1' held in contact with the neighboring intermediate lamina 2 farthest from the lamina 1 can be electrically connected with the conductor 3 on such lamina 2 farthest from the lamina 1 through the conductive bushing in the through-hole 4'.
In the assembled condition, since the first surface of only the outermost lamina 1' where the conductor 6' is formed is laid bare and exposed to the outside, a cover plate 7 similar in material and shape to any one of the laminas 1, 2 and 1' is placed over the outermost lamina 1' to conceal the conductor 6'.
All of these laminas including the cover plate are in practice clamped together, and the stack is then baked, or otherwise heat-treated, followed by the connection to terminal electrodes (not shown) to the respective opposite ends of the stack in electrically connected relationship with the strips 6b and 6'b on the associated outermost laminas 1 and 1'.
Thus, in the prior art multi-layer chip inductor, the conductors 3 on the respective intermediate laminas 2 and the conductors 6 and 6' on the respective outermost laminas 1 and 1' are connected electrically in series with each other through the conductive bushings 5 connecting the perforated end 3a of the conductor 3 on one intermediate lamina with the non-perforated end 3b of the conductor 3 on the neighboring intermediate lamina positioned therebelow while the conductive strip 6a of the conductor 6 on the outermost lamina 1 is connected with the perforated end 3a of the conductor 3 on the intermediate lamina 2 immediately thereabove through the conductive bushing 5 and the conductive strip 6'a of the conductor 6' on the outermost lamina 1' is connected with the non-perforated end 3b of the conductor 3 on the intermediate lamina 2 immediately therebelow through the conductive bushing in the through-hole 4'.
It has, however, been found that the prior art chip inductor of the construction shown in and described with reference to FIGS. 1 and 2 has the following problem. Specifically, where the wall thickness of the conductive bushing 5 situated inside each through-hole 4 or 4' is small, it often happens that some or all of the conductive bushings tend to evaporate and diffuse during the heat-treatment and the series connection of these conductors consequently breaks down.
In order to avoid the above mentioned problem, it is known to form each conductive bushing 5 plated, or otherwise lined, with another similar conductive bushing 5' as shown in FIG. 3 with one end of the conductive bushing 5' adjacent the second surface of the respective lamina 2 or 1' allowed to spread radially outwardly therefrom for facilitating a positive contact of the perforated end 3a of the conductor 3 or the conductive strip 6'a in one lamina with the non-perforated end 3b of the conductor 3 or the conductive strip 6a in the next adjacent lamina immediately therebelow. The use of the double-walled conductive bushing in each of the laminas such as shown in FIG. 3 brings about such a disadvantage that since areas of electric series connection of the conductors are concentrated around the respective through-holes 4 and 4' and since the wall thickness of each double-walled bushing is increased, some or all of the magnetizeable laminas as well as some or all of the through-holes tend to be deformed when the stack is clamped, and/or any possible shortcircuiting is about to occur during the use which would result in the breakdown of the chip inductor.